Since this is a one-time "special" event, I see no problem in doing this. Curious to see what others say.
Just make sure you take plenty of "sign up" forms with you - I have a feeling you'll need them.
Do you know what you'll include on your sign up form (what information you'll collect)?
You might consider including a time/day that you can contact them for a "free" consult (worth $xx - this lets them know that this is a great deal, too - it's free to them, but you also assign a dollar value to it so it's
of value - hope this made sense, LOL) to discuss their needs. Something like: Free consultation valued at $xx.
You might consider package deals. They are a great way to offer a "discount" for your services by bundling services you feel PCs may find of help to them. A side note: sometimes with package deals you end up making more $$.